Not trying to be funny or anything but i think the answer to "how I can show passion?" is to simply have passion for flying first.
Perhaps you are asking how to articulate the passion... how to put it in words. I do not profess to be an expert here but maybe you could try this.
1.) Why do you want to be a pilot?
2.) Why is it important for you to achieve this goal of being a pilot?
3.) What do you want to achieve/accomplish from it?
Just keep asking the whys, delve deeper... built the conviction why there is nothing else you'd rather do than being a pilot... and say it out in the interview.
Is there a deeper reason why you want to be a pilot? Is it because some uncle that you hold in high esteem was a pilot and you want to follow his footsteps? Is it because you have a dream of flying all over the world, get out of this tiny island?
The deeper you can answer the 'whys' the further it will bring you when times are hard. Not just during the interview..
If money is the answer to the first question, then why is it important? To buy a big house? Why is buying a big house important then? To show that you are successful? Then why is being successful important? It can be a bit irritating but once you get to the root of the 'why', i think you will find it easy to show the passion.